On-line measuring technology preferably makes use of NIR spectrometry, which is capable of operating rapidly and also without contact. However, for some applications this wavelength range is not suitable, e.g. if carbon black or graphite are used as pigments, when the method is to be surface sensitive or when foreign atoms, such for example as nitrogen, are to be recognized. In these cases operation must be at wavelengths of greater than 2.5 .mu.m, i.e. in MIR. In this range Fourier transformation infra-red (FTIR) spectrometers are the means used until now for selection. By means of FTIR spectrometers however only full spectra can be recorded over a wide wavelength range of about 2.5-25 .mu.m. This requires a relatively long measuring period. In this wavelength range also the room temperature background radiation (about 10 .mu.m), is found, whose influence can only be suppressed by expensive measures such as cooled diaphragms and filters or by modulation of the illuminating radiation. FTIR spectrometers also use moving parts, so that their range of applicability is restricted. Furthermore it is difficult to use them at a large distance from the substances to be investigated. Finally, their manufacturing costs are also extremely high.
A method for routine identification of the material of plastic parts with the aid of infra-red spectroscopy is already known from DE 43 40 914 A1. In this case an infra-red reflection spectrum is recorded from the surface of a plastic part to be investigated, and is compared with a set of reference spectra. The reflection spectrum used lies in the MIR range at a wave number range between 400 and 4000 cm.sup.-1. This area still partly lies in the range of heat radiation. In order that this has no disturbing effect on the measuring procedure, it is necessary to modulate the radiation before it impinges on the plastic part. According to DE 43 40 914 A1, an interferometer is used for this purpose, in which the radiation emitted by an IR light source is subjected to intensity modulation. The optical transmission of such an interferometer is however limited and the power of its output radiation is low in relation to the power of its input radiation. Therefore the output radiation lies at only a few watts, so that the signal-to-noise ratio is low. In order however to be able to investigate carbon-containing plastics in a routine manner by radiation in the MIR range, the document DE 43 40 914 A1 proposes to position the plastic part to be investigated with the aid of a video device. In this way it can be assured that the plastic part is located in a correct measuring position.
Thus however the known method has the disadvantages that it requires an expensive device for positioning, and that it operates relatively slowly due to the positioning procedure.
If a surface irradiator is used as a radiation source, it can be operated at a maximum temperature of 1000.degree. C. Therefore an increase in power can only be achieved by corresponding enlargement of its surface area. This is difficult in the case of a subsequently-incorporated interferometer, so that for this reason the radiation power is restricted to a maximum of 50 watts.